A portable device (“PD”), such as a smart phone, a laptop or notebook computer, and a cellular handset (just to name a few) is of course battery operated and therefore needs to be coupled to an external power source (“EPS”) to charge the battery. Typically, a PD has a battery charging circuit that draws current from a power pin (power line) of a communications interface connector of the device. For example, the current needed to charge the battery may be drawn from the Vbus pin of a Universal Serial Bus (USB) connector, while the latter is connected to a USB power adapter or to a desktop personal computer's high power USB port. The USB connector also has a data pin (data line), more specifically a pair of differentially driven data lines, used to primarily transfer data, rather than power, between the PD at one end and another computing device that is connected to the other end of a USB cable.
As PDs evolve with greater power consumption and larger battery capacity, the amount of current drawn from the EPS while charging the battery rises, for instance to one ampere or more. In addition, industry recommended requirements for communications interfaces (that also are power conduits) place an upper limit on the dc voltage of the power line that is close to the battery voltage. For example, a Vbus specification of 5 Volts dc at the output port of the USB power adapter circuit is close to the cell voltage of a fully charged lithium polymer cell, namely about 4.2 Volts.